EP2194546A1

EP2194546A1 - Electrical machine with improved lightning impulse withstand

Info

Publication number: EP2194546A1
Application number: EP08170944A
Authority: EP
Inventors: Jan Hajek; Arne Hjortsberg
Original assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Current assignee: ABB Research Ltd Switzerland; ABB Research Ltd Sweden
Priority date: 2008-12-08
Filing date: 2008-12-08
Publication date: 2010-06-09
Also published as: WO2010066710A1; BRPI0922679A2; CN102239532A; EP2374140A1; US20110298573A1

Abstract

The present invention relates to an electrical machine with a vertical axis. The machine comprises a winding arranged in a plurality of substantially circular turns wound around the axis. Wedge formed areas are formed between two adjacent turns of the winding. A member of insulating material is arranged to cover at least one of the wedge formed areas.

Description

FIELD OF INVENTION

The present invention relates to electrical machines, and particularly to an electrical machine with improved electrical withstand to transients.

BACKGROUND

In electrical machines, due to the configuration of the winding, strong electrical fields occur locally with impulse voltages. In such areas there is an increased risk for an electrical breakdown in the cooling medium being in contact with the winding.
A critical area is the wedge formed area between two adjacent turns of a winding in a winding in a power transformer, an area often exposed to locally strong electrical fields. Due to the difference of permittivity between oil and paper insulation, conventionally used in power transformers as cooling medium and insulation, respectively, there is an increased risk of electrical breakdown of the oil in such areas.
When the winding is submitted to impulse voltages, the electric field stress in such a wedge is high which can initiate a partial breakdown of the oil, which might enable a streamer (spark) to propagate via the oil to a surface, i.e. a barrier, and to continue propagating to ground. If such streamers occur, they will cause serious damages to the transformer. To prevent the occurrence of streamers, an electrical machine like a power transformer needs to be dimensioned to cope with high electrical stress occurring locally in such areas as described above. As a result, power transformers are dimensioned to withstand electrical field strength being several times stronger than normal workload, thus resulting in costly, large and bulky machines.
US 5,969,456 deals with a similar problem of electrical discharges, namely the problem of impulse voltage being unequally shared by winding portions in induction machines, tending to result in dielectric breakdown in a winding portion sharing excessively high voltage. To overcome this problem, an electromagnetic equipment having improved corona resistance and dielectric strength is described. In the known equipment, an electromagnetic winding is formed by winding an insulated wire comprising a cable having a conductor covered by an inner layer of insulating coating, and an outer low-resistance conductive layer of coating, the outer coating having a lower resistivity than the inner coating. By such arrangement, electric charges discharged by corona discharge can be restrained by the outer coating formed on the conductor. The conductive coating of each unit winding thus improves the corona resistance of each winding unit to ground. Further, the outer coating increases a series capacitance between winding conductors. Consequently, the voltage is equally shared by the windings of the electromagnetic winding, and the occurrence of strong electrical fields locally is reduced. As a result, improved insulation strength of the electromagnetic equipment relative to impulse voltages is achieved.
Although providing a winding with improved electrical withstand, a disadvantage with the known equipment is that such a winding provided with several layers of insulating material will be bulky and difficult to handle during winding of the machine. As a result, a machine provided with such a winding will be large and voluminous. Further, when the winding is wound in turns, there will still be a higher electrical field in the wedge between two adjacent layers, as described above.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electrical machine overcoming the problem mentioned above with propagation of streamers in local areas exposed to strong electrical fields.
This object, among others, is according to the present invention attained by an electrical machine, as defined by the appended claims.
According to the present invention, an electrical machine with improved lightning impulse withstand is provided. The machine comprises a vertical axis, and a winding arranged in a plurality of substantially circular turns wound around the axis. Wedge formed areas are formed between two adjacent turns of the winding. At least one member of insulating material is arranged to cover at least one of the wedge formed areas.
According to one embodiment of the invention, the winding comprises at least two consecutive turns being arranged essentially in a first plane, followed by at least two consecutive turns being arranged essentially in a second plane. The first and second planes are perpendicular to the vertical axis. The at least one wedge formed area is formed in the periphery of the winding between the two planes.
According to another embodiment of the invention, the at least one wedge formed area is located in the outer periphery of the winding.
According to yet another embodiment of the invention, the at least one wedge formed area is located in the inner periphery of the winding.
According to another embodiment of the invention, the winding is wound essentially in more than two consecutive planes each being perpendicular to said vertical axis, and wherein said at least one member of insulating material is arranged to cover at least one wedge formed area being formed between two consecutive planes, for which planes the turns shift from one plane to the other plane in the inner periphery of the winding.
According to another embodiment of the invention, the winding is wound essentially in more than two consecutive planes each being perpendicular to the vertical axis, and wherein the at least one member of insulating material is arranged to cover said at least one wedge formed area being formed between two consecutive planes, for which planes the turns shift from one plane to the other plane in the outer periphery of the winding.
According to another embodiment of the invention, the member of insulating material is arranged in contact with the adjacent turns.
According to another embodiment of the invention, the member of insulating material is arranged to cover a plurality of the wedge formed areas.
According to another embodiment of the invention, the member of insulating material is an insulating paper.
According to another embodiment of the invention, the member of insulating material is a thin barrier.
According to another embodiment of the invention, the electrical machine is a power transformer.
According to another embodiment of the invention, the power transformer is chilled by a liquid, and the at least one wedge formed area is in contact with the liquid.
According to another embodiment of the invention the liquid is oil.
According to another embodiment of the invention, the liquid is ester fluid.
According to yet another embodiment of the invention, the power transformer is a 3-phase power transformer.
Further features and advantages of the present invention will be evident from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description of embodiments given below and the accompanying figures, which are given by way of illustration only, and thus, are not limitative of the present invention, wherein:

Fig. 1a is a detailed drawing showing streamers occurring between two turns in a winding in a conventional electrical machine.
Fig. 1b is a detailed drawing showing two turns in a winding in an electrical machine according to the present invention.
Fig. 2 is a cross-sectional view of an electrical machine according to the present invention.
Fig. 3 is a top view of an electrical machine according to the present invention.
Fig. 4a is a detailed drawing showing two turns in a winding in an electrical machine with an isolating member according to one embodiment of the present invention.
Fig. 4b is a detailed drawing showing two turns in a winding in an electrical machine with an isolating member according to another embodiment of according to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, for purpose of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent for a person skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed description of well-known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details.
An embodiment of an electrical machine according to the present invention will now be described with reference to Figs. 1-4.
Fig. 1a is a detailed drawing showing streamers propagating between two turns 3 in a winding in an electrical machine, such as a power transformer. The electrical machine comprises a winding 2 with one, or a plurality of, insulated conductors wound in a plurality of turns 3 around a vertical axis 1. Between two adjacent turns 3 in the outer periphery of the winding 2, wedge formed areas 4 are formed. Due to the configuration of the winding 2, strong electrical field will occur locally in the wedge formed area 4. The difference of permittivity between oil and paper insulation, often used in power transformers as cooling medium and insulation, respectively, increases the risk of electrical breakdown of the oil in such areas whereby an initiated streamer may be enabled to propagate through the oil to ground. In the figure, a streamer 11 propagating through the oil via a barrier 10 to ground is shown. Alternatively, a streamer 11 might be able to propagate directly to ground through the oil.
Fig. 1b is a detailed drawing showing the corresponding two turns 3 in a winding 2 in an electrical machine according to the present invention. The electrical machine comprises a winding 2 with one, or a plurality of, insulated conductors wound in a plurality of turns 3 around a vertical axis 1. The winding 2 can be wound around a core or simply around the vertical axis 1, so called air-cored winding such as the one used in an oil filled reactor. Between two adjacent turns 3 in the outer periphery 8 of the winding 2, wedge formed areas 4 are formed. Strong electrical field will occur locally in the wedge formed area, increasing the risk of propagating streamers. A member of insulating material 5 is arranged to cover the wedge formed areas. Charges occurring in the wedge formed area 4 are thereby trapped in the wedge and are not able to develop into propagating streamers. By such an arrangement, improved lightning impulse withstand is provided. An electrical machine, such as a power transformer or reactor, thereby better withstand electrical stress due to strong electrical fields and the risk of propagating streamers casing severe damages are limited.
Fig. 2 shows a cross-sectional view of details in an electrical machine according to one embodiment of the present invention. The machine comprises a core 7 with a vertical axis 1, and a winding 2 arranged in a plurality of substantially circular turns 3 wound around the core 7 to form an electromagnetic winding. The winding 2 comprises four consecutive turns 3a, 3b, 3c, 3d arranged in a first plane 6a, followed by four consecutive turns 3e, 3f, 3g, 3h arranged in a second plane 6b, further followed by four consecutive turns 3i, 3j, 3k, 31 arranged in a third plane 6c and further followed by four consecutive turns 3m, 3n, 3o, 3p arranged in a fourth plane 6d.
The planes 6a, 6b, 6c and 6d are perpendicular in relation to the vertical axis 1 of the core 7. The machine further comprises a barrier 10 around the core 7 and the winding 2.
As seen in the figure, in the outer periphery 8 of the winding 2, there are seven intermediate turns (turns 3a-3g) between planes 6a and 6b, while there is only one turn in the outer periphery between planes 6b and 6c (turns 3h and 3i). Therefore, the electrical field in the wedge formed area 4a will be much stronger than the electrical field in the wedge formed area 4b, thereby increasing the risk of propagating streamers in the wedge 4a with damages as a result. A member of insulating material 5a is arranged to cover the wedge formed area 4a, whereby charges are trapped in the wedge and prevented from developing propagating streamers. Further, in the inner periphery 9 of the winding 2, there are seven intermediate turns (turns 3e-3h) between planes 6b and 6c. Therefore, the electrical field in the wedge formed area in the inner periphery 9 between plane 6b and plane 6c will be strong and a member of insulating material 5b is arranged to cover the wedge formed area.
Fig. 3 shows details seen from above of an electrical machine according to one embodiment of the present invention. The machine comprises a core 7 with a vertical axis 1, and a winding 2 arranged in a plurality of substantially circular turns 3 wound around the core 7 to form an electromagnetic winding.
The winding 2 comprises four consecutive turns 3a, 3b, 3c, 3d arranged in a first plane 6a, followed by four consecutive turns 3e, 3f, 3g, 3h arranged in a second plane 6b (not shown). The first and second planes 6a, 6b are perpendicular in relation to the vertical axis 1 of the core 7. The machine further comprises a barrier 10 around the core 7 and the winding 2. A first member of insulating material 5a is arranged to cover the wedge formed area 4 (not shown) in the outer periphery 8 between the first and the second plane. Further, a second member of insulated material 5b is arranged to cover the wedge formed area 4 (not shown) in the inner periphery 9 between the first and the second plane. Thereby, the risk of propagating streamers occurring in the machine is reduced in an efficient way.
The insulating material (5) can be one or more layers of isolation paper normally used in oil-filled electrical machines or other types of cellulose based materials, such as presspan or pressboard. The insulating material can also be a polymer, for example, Nomex, a meta-aramid material.
Fig. 4a and 4b shows examples of different forms of the isolating member 5. The member of insulating material 5 can have any practical form that prevents the propagation of streamers. In fig 4a the isolating member 5 is arranged with a wedge that fits into the wedge formed area 4 between two adjacent turns 3. In fig 4b the isolating member 5 is only a wedge that will fill the wedge formed area 4 between two adjacent turns 3.
It will be obvious that the present invention may be varied in a plurality of ways. Such variations are not to be regarded as departure from the scope of the present invention as defined by the appended claims. All such variations as would be obvious for a person skilled in the art are intended to be included within the scope of the present invention as defined by the appended claims.

Claims

An electrical machine with a vertical axis (1) comprising;
- a winding (2) arranged in a plurality of substantially circular turns (3) wound around said axis,
wherein wedge formed areas (4) are formed between two adjacent turns (3) of said winding,
characterized in that
at least one member of insulating material (5) is arranged to cover at least one of said wedge formed areas.
The electrical machine according to claim 1,
wherein said winding (2) comprises at least two consecutive turns (3) being arranged essentially in a first plane (6a), followed by at least two consecutive turns being arranged essentially in a second plane (6b), said first and second planes being perpendicular to said vertical axis (1),
and wherein said at least one wedge formed area (4) is formed in the periphery of said winding (2) between said two planes.
The electrical machine according to claim 1 or 2,
wherein said at least one wedge formed area (4) is located in the outer periphery (8) of said winding (2).
The electrical machine according to claim 3, wherein said winding (2) is wound essentially in two or more consecutive planes (6) each being perpendicular to said vertical axis (1), and wherein said at least one member of insulating material (5) is arranged to cover at least one wedge formed area (4) being formed between two consecutive planes (6), for which planes the turns (3) shift from one plane to the other plane in the inner periphery (9) of said winding.
The electrical machine according to claim 1 or 2,
wherein said at least one wedge formed area (4) is located in the inner periphery (9) of said winding (2).
The electrical machine according to claim 5, wherein said winding (2) is wound essentially in two or more consecutive planes (6) each being perpendicular to said vertical axis (1), and wherein said at least one member of insulating material (5) is arranged to cover said at least one wedge formed area (4) being formed between two consecutive planes, for which planes the turns (3) shift from one plane to the other plane in the outer periphery (8) of said winding.
The electrical machine according to any of claims 1-5,
wherein said member of insulating material (5) is arranged in contact with said adjacent turns (3).
The electrical machine according to any of claims 1-7,
wherein said member of insulating material (5) is arranged to cover a plurality of said wedge formed areas (4).
The electrical machine according to any of claims 1-8,
wherein said member of insulating material (5) one or more layers of insulating paper.
The electrical machine according to any of claims 1-8,
wherein said member of insulating material (5) is a thin barrier of Nomex.
The electrical machine according to any of claims 1-8,
wherein said member of insulating material (5) is a thin barrier of presspan.
The electrical machine according to any of claims 1-11,
wherein said electrical machine is a reactor.
The electrical machine according to any of claims 1-11,
wherein said electrical machine is a power transformer.
The electrical machine according to claim 13,
wherein said power transformer is chilled by a liquid, and
wherein said at least one wedge formed area (4) is in contact with the liquid.
The electrical machine according to claim 14,
wherein said liquid is oil.
The electrical machine according to claim 14,
wherein said liquid is ester fluid.
The electrical machine according to any of claims 13-16,
wherein said power transformer is a 3-phase power transformer.